1. Field of the Invention
The present invention relates to methods for manufacturing a diffraction optical element.
2. Description of the Related Art
Hitherto, as a method for compensating for chromatic aberration, a method has been known in which optical elements composed of glass materials having different scattering properties were combined together. In addition, in SPIE International Lens Design Conference, vol. 1345, pp. 24 to 29, 1990, a technique for compensating for chromatic aberration by using a diffraction optical element was disclosed.
As a method for manufacturing the diffraction optical element described above, a method has been used, which comprises the steps of preparing a molding die having a pattern corresponding to a diffraction grating, transferring the pattern to a molding material, and then removing the molding material from the molding die. As the molding material described above, photocurable resins have been widely used.
In addition, as a method for forming a pattern on the surface of the molding die, heretofore, a method has been known which uses a typical microfabrication technique, such as a photolithography technique primarily used in a semiconductor manufacturing process, or an accurate cutting technique using a diamond bit.
However, when a photocurable resin is used as the molding material, due to a large shrinkage factor thereof caused by curing, there have been problems in that concave and convex portions, which form a diffraction grating, are poorly transferred and that sufficient structural accuracy cannot be obtained. In addition, an internal stress is liable to remain in the photocurable resin, and as a result, elements may be deformed by this stress in some cases.
In order to solve the problem of the insufficient structural accuracy described above, for example, a method has been disclosed in Japanese Patent Laid-Open No. 6-59104. In this publication, when a photocurable resin is cured, a region irradiated with light is gradually increased from a central portion of an optical element to the periphery thereof while the exposure is controlled by using a shutter. The method, however, is not for manufacturing a diffraction optical element. Nevertheless, according to this method, while the resin is gradually cured from the center, the shrunk portion of the cured resin is compensated for with the feed of an uncured resin. Thus, the decrease in structural accuracy can be prevented.
However, according to the method disclosed in Japanese Patent Laid-Open No. 6-59104, in the case in which a diffraction optical element provided with a fine pattern on the surface thereof is manufactured, a satisfactory effect could not be obtained.